Can I replace the 11 speed 11-40t cassette with a 11-42t or 11-46t? What other parts will be needed to complete job? I.e. new chain, long chain, spacers?
Depends on what derailleur and front rings you have.
The current Deore XT M8000 GS (which we can probably assume you have) specs say:
- Max 11-42T cassette for a 2x11 setup
- Max 11-46T cassette for a 1x11 setup
Additionally for a 2x11 setup there is a max 10 teeth difference between the front rings.